1. Field of the Invention
The present invention relates to a print element substrate including a plurality of print element arrays in which different numbers of print elements are arrayed, an inkjet printhead, and a printing apparatus.
2. Description of the Related Art
A printhead which prints on a printing medium by discharging ink according to a thermal inkjet method includes, as print element building elements in the printhead, heaters formed from heat generation elements. Drivers for driving heaters, and logic circuits for selectively driving the drivers in accordance with print data are formed on a single element substrate of the printhead.
The resolution of thermal inkjet type color inkjet printing apparatuses is increasing year by year. Along with this, the orifice arrangement density of a printhead is set to discharge ink in the range of a resolution of 600 dpi to resolutions of 900 dpi and 1,200 dpi. There is known a printhead having orifices at such high density.
Demand has arisen for reducing graininess at a halftone portion or highlight portion in a gray image and color photo image. To meet this demand, the size of an ink droplet (liquid droplet) discharged to form an image was about 15 pl several years ago, but is recently decreasing to 5 pl and then 2 pl year after year in a printhead which discharges color ink.
A high-resolution printhead in which orifices for discharging small ink droplets are arranged at high density satisfies a user need for high-quality printing when printing a high-quality color graphic image or photo image. However, when not high-resolution printing but high-speed printing is required in, for example, printing a color graph in a spreadsheet, the above-mentioned printhead may not meet the demand for high-speed printing because printing with small ink droplets increases the number of print scan operations.
To achieve even high-speed printing, there has been proposed a printhead which discharges small ink droplets for high-quality printing and large ink droplets for high-speed printing. There have also been known a printhead in which a plurality of heaters are arranged for one orifice to change the discharge amount by these heaters, and a printhead in which a plurality of orifices having different discharge amounts are arranged in one element substrate.
Element substrates having a plurality of orifices for discharging different amounts of ink include an element substrate in which an orifice array (small-droplet orifice array) of orifices for discharging small ink droplets, and an orifice array (large-droplet orifice array) of orifices for discharging large ink droplets are juxtaposed. To achieve high-quality printing at high speed by this element substrate, there is proposed an element substrate in which the orifice arrangement density of a small-droplet orifice array is higher than that of a large-droplet orifice array. An example of this element substrate is one having a large-droplet orifice array in which 600 orifices are arranged per inch (arrangement density is 600 dpi), and a small-droplet orifice array in which 1,200 orifices double in number are arranged per inch (arrangement density is 1,200 dpi).
To stably discharge ink, a stable voltage needs to be applied to heaters. When all heaters are driven concurrently, a large current flows, and the voltage greatly drops owing to the wiring resistance. To solve this, there is a time-divisional driving method of dividing a plurality of heaters on an element substrate into a plurality of blocks, and sequentially driving heaters for the respective blocks time-divisionally to stably discharge ink. Examples of this element substrate are arrangements disclosed in the U.S. Pat. Nos. 6,409,315, 6,474,790, 5,754,201, 6,137,502, and 6,966,629, and Japanese Patent Laid-Open No. 2002-374163.
Recent inkjet printing apparatuses discharge small ink droplets to print a high-quality image. At the same time, these inkjet printing apparatuses need to increase the print speed. Simply forming the same image requires the same ink amount. Thus, if the discharged ink droplet is downsized to decrease the discharged ink amount to ½, the print speed simply decreases to ½.
To discharge the same ink amount in the same time in order to prevent a decrease in print speed, the number of heaters needs to be doubled. If the number of heaters is doubled without changing the heater arrangement density, the size of an element substrate in which heaters are arranged increases double or more. In addition to the increase in element substrate size, this also increases the size of the printhead which moves at high speed in the printing apparatus, the size of the printing apparatus, and vibrations and noise. To prevent these, the heater arrangement density needs to be increased.
To print at high speed, a printhead having orifices for discharging large ink droplets is more advantageous than one having only orifices for discharging small ink droplets. Recent inkjet printing apparatuses adopt a printhead having an element substrate in which a small-droplet orifice array and large-droplet orifice array are juxtaposed. These inkjet printing apparatuses achieve both high-speed printing and high-quality printing by selectively driving orifices for discharging small ink droplets and those for discharging large ink droplets. However, to implement both high-speed printing and high-quality printing, the numbers of orifices and heaters integrated on the element substrate need to be increased.
An element substrate in which the orifice arrangement density of a small-droplet orifice array is higher than that of a large-droplet orifice array will be described. As this element substrate, an element substrate including a large-droplet orifice array at an arrangement density of 600 dpi and a small-droplet orifice array with a double number of orifices at a double arrangement density of 1,200 dpi, which are arranged on a single substrate, will be exemplified. In this element substrate, when printing one pixel by one bit, the number of heaters directly equals the number of bits of print data. The data amount necessary for the orifice array at the arrangement density of 1,200 dpi is double the data amount necessary for the orifice array at the arrangement density of 600 dpi.
As an example of the time-divisional driving method, the time-divisional counts of respective orifice arrays on an element substrate are sometimes set equal to each other. Since a common clock is used within the element substrate, the clock frequency is the same. For this reason, the data transfer speed is proportional to the number of bits in a shift register used to transfer data. Data is transferred for each orifice array, so the time taken for data transfer differs between a high-density orifice array and a low-density orifice array. For example, assume that the number of bits in a shift register corresponding to an orifice array made up of a relatively small number of orifices is 6 bits (4 bits for print data and 2 bits for block control data). Also assume that the number of bits in a shift register corresponding to an orifice array made up of a relatively lager number of orifices is 10 bits (8 bits for print data and 2 bits for block control data). In this case, even the data transfer speed for the 6-bit shift register is limited by that for the 10-bit shift register. Therefore, the 6-bit shift register transfers data at 6/10 of the original data transfer speed.
There is also a method of increasing the frequency of a clock for transferring print data in order to print at high speed. In general, the clock is supplied from the printing apparatus main body to the printhead. The printhead which moves during printing, and the printing apparatus main body are connected by a relatively long cable such as a flexible cable. Since this cable contains plural signal lines and current supply lines, noise is readily superposed on signals transmitted through the cable. The inductance component of the cable delays the rise and fall of the pulse waveform (distorted waveform). This becomes non-negligible because, as the clock cycle shortens, the ratio of fluctuations becomes relatively high. Thus, the printhead may malfunction. When a signal is transmitted using a high-frequency clock, the cable may function as an antenna to generate radiation noise. The radiation noise may cause the malfunction of a peripheral device.